1. Field of the Invention
This invention relates to a device and a method for multiplexing a plurality of bit streams and also to a device and a method for decoding a multiplexed signal. The present invention also relates to a recording medium storing a program for multiplexing a plurality of bit streams.
2. Description of the Related Art
In systems such as video conference systems and video phone systems adapted to transmitting moving picture signals to remote receivers, video signals are normally compressed and encoded by utilizing line correlations and interframe correlations of picture signals in order to efficiently utilize the transmission path.
The MPEG (Moving Picture Experts Group) system (for coding moving pictures for storage) is a typical highly efficient coding system for processing moving pictures. This system was adopted as standard system as a result of the discussions in the ISO-IEC/JTC1/SC29/WG11. As a matter of fact, it is a hybrid system obtained by combining motion compensation predictive coding and discrete cosine transformation (DCT) coding.
Assume now that a picture is taken by means of a first video camera 101 and a second video camera 102 as shown in FIG. 1 of the accompanying drawings and encoded by means of such a coding system.
Also assume that the first bit stream BS1 output from the first video camera and the second bit stream BS2 output from the second video camera are synchronized on a frame by frame basis for the two obtained pictures and multiplexed into a single bit stream BSMUX by multiplexing section 103.
Then, conventionally, the so-called MPEG 2 system (ISO/IEC19818-1) is used for the multiplexing. This multiplexing technique will now be described by referring to FIG. 2 of the accompanying drawings.
FIG. 2 schematically illustrates how two bit streams of encoded pictures are coded simultaneously on a frame by frame basis for the two obtained pictures so that they may be synchronized.
In FIG. 2, A schematically shows the first bit stream BS1 while B schematically shows the second bit stream BS2. Each box of the first and second bit streams BS1, BS2 represents the smallest unit for forming a bit stream that is referred to as access unit (AU). In the case of MPEG 2, an access unit corresponds to a single picture.
In the example of FIG. 2, the frame unit of the first bit stream BS1 and that of the second bit stream BS2 shows a dimensional ratio of 2:1. In other words, the time of two access units of the first bit stream BS1 corresponds to that of a single access unit of the second bit stream BS2.
In the case of MPEG 2, the input bit stream is divided into access units having an arbitrarily selected time. For the purpose of simplicity, assume here that the first bit stream is divided into access unit AU10, access unit AU11, access unit AU12, access unit AU13, . . .
Also assume that the bit stream BS2 is divided into similar access units or frames. More specifically, it is divided into access unit AU20, access unit AU21, access unit AU22, access unit AU23, . . .
Thereafter, as shown in C and D of FIG. 2, PES (packetized elementary stream) packets are formed by adding PES packet headers to the divided bit streams on the basis of a given time unit good for both of the bit streams.
A PTS (presentation time stamp) that is time control information is inserted into each PES packet header. The display time information of the leading access unit is written into the PTS. Any corresponding PES packets are made to have a same PTS.
In this way, PES packets are formed for the first and second bit streams BS1 and BS2 that are divided. Then, the first and second bit streams BS1 and BS2 are transformed into a single bit stream on the basis of their PES packets.
Thus, according to the MPEG 2 system, a program stream (PS) for forming a single program or a transport stream (TS) for forming a plurality of programs are generated by using PES packets.
In this way, it is possible to multiplexes a pair of bit streams to form a single bit stream, synchronizing them by using the MPEG system.
However, when multiplexing a pair of bit streams by using the MPEG 2 system, it is necessary to inversely multiplex a multiplexed bit stream BSMUX to rearrange the bit stream and decode the coded signal in order to restore the original bit streams from the multiplexed bit stream BSMUX.
In other words, when decoding the multiplexed bit stream BSMUX, there are needed a means for rearranging the inversely multiplexed bit stream and a means for decoding the rearranged bit stream.
Therefore, conventionally, the process of decoding a multiplexed bit stream BSMUX requires the use of complex devices and decoding steps.